During the past few years, the concept of stealth technology has been successfully exploited, especially for aircrafts. In its most basic definition, stealth is the art of going un-noticed through an environment. The aim is therefore to make it increasingly difficult to detect an object by means of e.g. radar or other electromagnetic detection technique. A plurality of designs, materials, and electronic devices has therefore been developed for this purpose.
Major potential sources of high radar visibility in stealth objects are antennas associated with the object. Since an antenna is typically designed to absorb energy in its operational band, the in-band diffraction is significant if the antenna is integrated in a non-absorbing environment. The out of band diffraction can also contribute to the so called radar cross section (RCS) if there is a phase difference between the reflection from the antenna and the reflection from the surroundings. Several phenomena have been identified as contributions to the radar visibility as represented by the radar cross section (RCS) of array antennas. These contributors can be divided according to: i) structural RCS, ii) antenna-mode RCS, i.e. reflections from inside the antenna, and iii) grating lobes i.e. above radio frequency (RF) band spikes. Examples of the various “classes” of contributors are e.g. grating lobes, edge diffraction, and surface waves
The grating lobes can occur if the inter-element spacing is larger then half a wave length [[1, [2, [3].
Edge diffraction can be interpreted as diffraction caused by the rapid change in the scattering properties between the antenna and its surroundings [[4]. The out of band diffraction can also contribute to the RCS if there is a phase difference between reflections from the antenna and reflections from the antenna surrounding.
Therefore, there is a need for methods and arrangements to reduce the RCS of antennas.